1. Field of the Invention
The invention relates to a scanning diversity system for motor vehicles with a multi-antenna installation.
2. Description of the Prior Art
Circuit arrangements of this type are known in the prior art. For example, German Patent application DE 3517247 shows a diversity antenna designed to eliminate interferences with the reception of frequency-modulated (FM) radio broadcasts in the antenna. This diversity processor is for receiving a number of antenna signals which are switched through to the receiver at each point in time. A second signal is derived from this first signal in the receiver in the intermediate frequency range (IF) through transformation, and supplied to the diversity processor for the detection of interferences.
When interference in the system is detected, switching signals are derived in the diversity processor so that they switch to another antenna signal. Audio frequency interferences conditioned by multi-channel reception are then largely avoided so that the actual receive conditions are adequate. However, in regions where the reception conditions are poor, the circuit arrangement shown in German patent No. 3517247 are affected with frequently occurring changeover processes, such as interference caused by such stepped switching. In regions where the signals are weak, but the qualities of reproduction in the motor vehicle are good, the audio quality is impaired by these changeover interferences.
Frequency modulated (FM) signals such as an ultra-short wave (VHF) reception, or television audio receptions are subjected to reception interference while an antenna is switched on and operating. This is because of super positioning of a plurality of partial waves with varying differences in amplitude, phase and time delay in the location of reception. The breakdowns in level caused by these differences are correlated with upward peaks of frequency interference, and cause signal distortions depending on the content of modulation in the audio frequency range. Therefore, the minimum test time corresponds with the time in which the interference is detected. When interference is detected by the interference detector, the diversity processor induces further switching and tests the further signals and their linear combinations, if any, formed in the antenna matrix. If all available HF-signals show interferences, all HF-signals are continually switched through to the receiver successively. Since this search process is repeated in rapid sequence, any audible interference signal in the form of a crackling sound, superposes itself on the received signal at the low frequency (LF) level.
Therefore, German Patent DE 44003612 shows measures to limit the frequency of continued switching in regions where the reception is particularly unfavorable. This is shown through the use of a stop device. In cases where the diversity processor indicates that interference is occurring with excessive frequency, the stop device stops such continued switching with adaptation to the receive conditions. The interferences caused by change-over are in fact reduced by this measure. However, reception interferences indicated by the diversity processor are by nature reflected by effects of interference appearing in the receive signal.
Therefore, the object of the invention is to provide a scanning antenna diversity system by which continued switching is reduced and, as a consequence of such a reduction, interferences connected therewith are reduced as well.
The invention has the following advantages. First, the diversity processor selects the received signal supplying the highest level at any point in time. This availability is based on the preselection from the available received signals at any point in time, which means this received signal will consequently have interferences with lesser probability.
As a consequence, the benefits of the invention are that a received signal is selected wherein the probability of interference is less than the discrete signals, and the interference detection rate of a diversity processor is noticeably lower. On the other hand, it is known that extremely strong interferences may occur in FM radio broadcasting also in connection with large received signals. For example, the interferences are observed with multi-way reception where substantial differences exist with respect to time delay, or because of interferences in an adjacent or the same channel. Therefore, the diversity processor should also test the maximum level signal for freedom from interference, and in the affirmative, switches such a signal xe2x80x9conxe2x80x9d a discreetly present signal.